The third function of fan duct is to provide thrust augmentation
for the propulsion system.
Such a propulsion system called the inter-turbine burner (ITB), could enable power extraction, thrust augmentation
, reduced fuel burn, and specific thrust (ST) improvements.
Piper Aircraft is working with jet engine manufacturer Williams International to investigate a unique PiperJet thrust augmentation
system that could eliminate the need for its much-touted automatic horizontal stabilizer trim system to handle trim changes generated by the aircraft's single high-centreline FJ44-3AP turbofan engine.
The Jupiter-C did not use strap-on rockets; it had a set of rockets in the upper stage, but nothing that amounted to thrust augmentation
in the first stage.